Improving fuel economy is an important factor in developing lubricating oil compositions. In the automotive industry, manufacturers have required that engines oils have a demonstrable fuel economy. This can affect the new specifications for automotive lube oils. Engine oil formulators have employed appropriate additives to modify the composition's lubricity, typically with additives called friction modifiers/enhancers, selected to reduce the coefficient of friction of the lubricating oil composition.
Engine tests such as the Sequence VIB test have been introduced in the US to quantify fuel economy across a range of engine running conditions; Sequence VIB is considered as the reference test to predict fuel economy improvement resulting from lube oil formulations optimization. The Sequence VIB has five operational stages, each stage varies in engine speed, engine load and oil temperature. The response to an engine oil formulation is dependent on the engine operating conditions. At high speeds and low temperatures, a thick intervening oil film develops between moving engine parts. Under these conditions, the friction of the engine and hence fuel economy is highly dependent on the viscous drag of the engine oil. At low speeds and high temperatures, oil film breakdown can occur. In this case, friction is highly dependent on the adsorption of friction reducing additives on the surfaces of rubbing of engine parts. Minimizing overall engine friction thus maximizing Sequence VIB fuel economy requires selecting or tailoring additives to operate across these operational stages. There are also lab-scale tests such as the HTHS (High Temperature High Shear) Viscosity measurements which are described as giving indicators of fuel economy improvement.
U.S. Pat. No. 6,110,880 describes AB block copolymers having an ethylene (>93%) rich A block and a B block having an ethylene content between 40 and 85 wt %, a thickening efficiency of 2.3 and a SSI of less than 25% or a thickening efficiency of 3.5 and a shear stability less than 35%. The A block comprises 15 to 25 wt % of the block copolymer. The polymers are disclosed for use in lubricating oils to change the viscosity characteristics of the composition.
As known in the art, Viscosity Index Improvers “VII” (a term generally interchangeable with Viscosity Index Modifiers) reduce the rate of viscosity change with temperature and impart a non-Newtonian fluid property to the oil composition since the viscosity varies with the shear rate. While lowering the viscosity of engine oil is effective in reducing internal friction it also decreases the oil's film thickness and decreases the hydrodynamic boundary layer formation, which can lead to increased wear.
Generally shear stability is inversely proportional to molecular weight. A Viscosity Index Improver polymer with good shear stability (low SSI value) is typically used at higher initial concentrations relative to another polymer having reduced shear stability (high SSI value) to obtain the same target thickening effect in a treated fluid at high temperatures; the polymer having good shear stability may, however, produce unacceptable thickening at low temperatures due to the higher use concentrations. Conversely, although lubricating oils containing lower concentrations of reduced shear stability VI-improving polymers may initially satisfy the higher temperature viscosity target; fluid viscosity will decrease significantly with use causing a loss of effectiveness of the lubricating oil. Thus, the reduced shear stability of specific VI-improving polymers may be satisfactory at low temperatures (due to its lower concentration) but it may prove unsatisfactory under high temperature conditions. The amount of Viscosity Index Improver polymer necessary to achieve a given thickening effect is quantified and referred to as the thickening efficiency (TE).
The present invention is directed in part to the unexpected discovery that by carefully crafting the Viscosity Index Improving polymer's A and B blocks in relationship with the SSI, TE and NMR signal, the resulting polymer structure could be optimized for fuel economy improvement. It is among the objects of the invention to provide a Viscosity Index Improver polymer composition having molecular weight, thickening efficiency, shear stability and HTHS viscosity (High Temperature High Shear) viscosity with improved performance across a broader temperature range.